1. Field of the Invention
The present invention relates to a vehicular alternator mounted on a passenger car, a truck and the like.
2. Description of Related Art
A vehicular alternator receives motive energy from an engine and generates electricity. Thus, the alternator charges a battery and supplies the electric power to various electrical equipments such as an ignition device or lamps. In addition to reduction in size and weight or improvement in output, improvement in a lifetime of the product is also an important issue for maintaining and improving market competitiveness.
Generally, the vehicular alternator is often mounted in a lower space of an engine room because the vehicular alternator has relatively high mountability among accessories mounted to the engine. Moreover, size of the engine room is being reduced. Accordingly, removal of the vehicular alternator from the vehicle is becoming more and more laborious. Therefore, maintenance frequency of the vehicular alternator should be preferably reduced. Ideally, the vehicular alternator should be preferably made maintenance-free. To that end, lifetimes of brushes and collector rings need to be improved because the brushes and the collector rings are important factors determining the lifetime of the vehicular alternator. More specifically, abrasion resistance and environment resistance of the brushes and the collector rings need to be improved.
A technology disclosed in JP-A-S58-207841 (pages 4 and 5, FIGS. 1 to 6) (Patent Document 1) integrates an electrically conductive pipe member to a rotor shaft by a molding operation and the like and divides the integrated pipe member and the rotor shaft into a pair of collector rings. Thus, a general structure of the conventional collector rings can be obtained. The pair of collector rings providing a positive electrode and a negative electrode are made of the same material, since the single metal pipe is divided after the metal pipe is covered by a resin molding.
Copper is often used as the material of the collector ring. Stainless steel (SUS in the Japanese Industrial Standards) is also used as the material of the collector ring. In addition to electrical characteristics (voltage drop between the brush and a portion contacting the brush in a sliding manner) or mechanical characteristics (the abrasion resistance of the collector ring and the brush), economy (a material cost, a process cost) should be taken into account when the material of the collector ring is selected. Therefore, tendencies of correlations between the materials and the various characteristics are examined, for instance, in JP-A-H01-40592 (pages 3 and 4, FIGS. 1 to 5) (Patent Document 2). According to the examination in Patent Document 2, the copper has the better electrical characteristics than the stainless steel, and the cost of the copper is lower than the stainless steel. The stainless steel has the better mechanical characteristics than the copper. The copper or the stainless steel is selected and used in accordance with required characteristics.
Another structure of the collector ring is disclosed in JP-A-S52-144705 (pages 1 and 2, FIGS. 1 to 3) (Patent Document 3), JP-A-S52-144706 (pages 1 and 2, FIGS. 1 to 3) (Patent Document 4), and JP-A-S52-144707 (pages 1 and 2, FIGS. 1 to 3) (Patent Document 5). According to Patent Documents 3 to 5, an end of a rotor coil is extended and wound around a rotary shaft into a cylindrical shape. Thus, the end of the rotor coil is fixed to the rotary shaft. Then, a surface of the cylindrical shape is smoothed by a cutting work and the cylindrical shape is divided to form two metal rings. The metal rings are used as the collector rings.
Generally, the copper having the excellent electrical characteristics is used as the material of the collector ring having the structure disclosed in Patent Document 1. However, because of the mechanical characteristics of the copper, the surface of the collector ring tends to be abraded and tends to become rough due to heat generation accompanying the sliding contact between the collector ring and the brush made of metal graphite. Specifically, the abrasion and the roughness further tend to occur on the surface of the collector ring on a positive electrode side because of brush sparks. As a result, the abrasion of the collector rings and the brushes is promoted significantly, and durability is deteriorated.
As a measure to the above problems, the stainless steel can be used as the material of the collector rings, since the mechanical characteristics of the stainless steel are superior to those of the copper. However, the electrical characteristics of the stainless steel are inferior to those of the copper. More specifically, contact resistance between the brush and the collector ring is deteriorated and the voltage drop is increased if the collector ring is made of the stainless steel. Therefore, current fed to the rotor coil is reduced (current feeding is the main function of the collector ring), and the output of the vehicular alternator is deteriorated. Moreover, cost of the parts and product cost increase since the stainless steel is more expensive than the copper.
Since the collector ring having the structure disclosed in each one of Patent Documents 3 to 5 is formed by extending the end of the rotor coil, the collector ring is made of the same material as that of the rotor coil (generally, the copper). Therefore, negative effects caused in the case where the ring-shaped collector ring is made of the copper cannot be avoided. Moreover, softening refining of the copper wire material of the coil is performed in order to improve winding performance. Accordingly, there is a possibility that the abrasion of the collector ring is worsened. Therefore, it is not preferable to use the collector ring of Patent Documents 3 to 5 as the collector ring of the rotor of the vehicular alternator used under severe conditions.